Salt gradient solar ponds are inexpensive solar heat collector/storage devices having a salt gradient layer which serves as an insulator to enable solar heat to be collected and stored in the solution at the bottom portion of the pond. Sunlight penetrates the salt gradient layer and is absorbed throughout the pond solution, particularly at the bottom of the pond, heating the solution. Because the density of the salt gradient layer increases from its upper boundary to its lower boundary, the salt gradient layer prevents the heated solution at the bottom of the pond from rising to the pond surface. Thus, the solution at the bottom eventually becomes heated to a relatively high temperature.
Ideally, a salt gradient solar pond defines three separate vertical zones or layers in the pond, commonly referred to as the lower convective zone, the gradient zone and the upper convective zone. The transition regions between the zones are referred to as zone boundaries.
The lower convective zones, which is the lower-most zone, has a uniform salient concentration and a uniform temperature profile. The solar heat is collected in this layer.
In the gradient zone, the salt concentration and temperature vary across the depth of the zone. The salt concentration is strongest at the lower edge of the zone adjacent to the lower convective zone, and the temperature of the gradient zone is warmer at the bottom of the zone. Although it is not universally true, frequently both the temperature and salient concentration vary almost linearly across the gradient zone. The gradient zone serves as an insulating barrier because the hot salty solution at the bottom edge of the gradient zone is more dense than the cooler, less salty solution at the top of the zone.
The upper convective zone, which is located immediately above the gradient zone, has a uniform salt concentration and uniform temperature profile. This zone provides isolation from the compensation for minor turbulences, etc. induced at the surface of the pond.
With proper design and under ideal conditions, the different zones will remain separate and distinct over a long period. However, excessive surface turbulence can cause the depth of the upper convective layer to increase, thereby decreasing the size of the gradient zone with an attendant reduction in its insulating effectiveness. Excessive mixing of the pond could eliminate the gradient zone entirely. Minor changes in the salient concentrations due to zone boundary migration can also diminish the gradient zone where at some value it may become too thin to be of much value. Consequently, some form of maintenance is generally required.
One of the key problems of salt gradient solar ponds is the control of the size of the gradient zone. From a thermal viewpoint, it is desirable to maintain the thickness of the gradient zone relatively large. The thicker the gradient zone, the greater the insulating value of the zone, and the less heat that is lost from the bottom of the pond to the surface. However, the thicker the gradient zone, the less sunlight that reaches the bottom of the pond. Thus, there is less heating of the storage layer solution. For a given pond operating temperature, there is an optimal gradient zone thickness. In practice, the optimal thickness is almost always greater than 1 meter.
A problem that arises is that the equilibrium thickness of the pond is always much less than what is desired thermally. In a practical pond, once the gradient zone is established, one or both of the convecting zones will grow at the expense of the gradient zone. The zone boundaries move until the thickness of the gradient zone reaches an equilibrium value, and the equilibrium thickness is much less than one meter for practical pond operating temperatures.
Zone boundary migration imposes a maintenance penalty on the operation of salt gradient solar ponds. If left unattended for several months, the gradient zone becomes too thin to be very effective. To keep the gradient zone thickness at the optimal design value, the gradient zone must be continually repaired. This requires energy to power pumps as well as the addition of considerably more makeup salt to the pond than would be required if the zone boundaries could be maintained stationary. The potential market for salt gradient ponds could be significantly expanded if this maintenance problem could be eliminated, so that salt gradient solar ponds could run unattended for long periods of time.
Laboratory experiments have demonstrated that zone boundary migration in salt gradient ponds is caused by turbulent entrainment into the gradient zone due to convective motions in both the lower and upper convective zones. See article by P. F. Linden, entitled "The Deepening of a Mixed Layer in a Stratified Fluid", Journal of Fluid Mechanics, Volume 71, pages 385-405 (1975). One approach for preventing this entrainment is to stretch transparent horizontal impermeable membranes across the pond at the zone boundaries. Salt gradient solar ponds incorporating such arrangement are disclosed, for example in U.S.S.R. Pat. No. 30827 to G. Ya. Umarov, R. A. Zaknidov, and Yu. U. Usmanov. Although this method solves the problem of turbulent entrainment, this method does not achieve the desired outcome because the membranes do not allow the transport of salt across them. Consequently, convective zones form at the top and bottom of the gradient zone on the gradient zone side of the membranes for the same reason that convective zones form in the convective layers at the top and bottom of the pond. The gradient zone itself will break up into a central smaller gradient zone which is located between upper and lower conductive zones created immediately adjacent to the gradient zone barriers.
Another approach is disclosed in Argonne National Laboratories Report. No. ANL-CT-80-23, of Sha et al., entitled "Some Basic Considerations and Possible Improvements on the Solar Pond". In this approach, a transparent honeycomb barrier is located in the gradient zone. The barrier has vertically open conductive passages or cells which extend between the lower and upper convective zones. The vertical height of the honeycomb barrier determines the height of the gradient zone and once the barrier is fabricated it cannot be varied. Moreover, the barrier is three dimensional and quite complex, making it difficult and expensive to fabricate and to support extended open uniformly across the width of the pond.